soap plant, any of various plants having cleansing properties. A few are of commercial importance, but most soap plants are used locally, as in early times, for toilet and laundry purposes. The soapbark (now often included in hair tonics) and the soapberry have been particularly valued for shampooing, and the California soap plant, the soapbark, and the soapwort for washing delicate fabrics. Soap plants contain no alkali and are considered mild and beneficial for cleansing purposes, with the exception of the soapberry, which is thought to harm some textile materials. The lather-producing substance is saponin, often poisonous if taken internally. This poisonous quality has been utilized by indigenous peoples, who have caught fish by first stupefying them with bits of the plants thrown into pools. There are many plants that are saponaceous, but only a few are known to contain appreciable amounts of saponin. The dried inner bark of the soapbark tree (
Quillaja saponaria) of the rue family, native to the Andes, has been collected also for commercial use in fire-extinguishing solutions and as an emulsifying agent for medicines and tars. New World and Old World species of soapberry (genus
Sapindus) provide saponin from the fruits. Since antiquity,
S. mukorossi has been used in E Asia and the Himalayas as a detergent for shawls and silks and by jewelers for cleaning silver. The soapwort, or bouncing Bet (
Saponaria officinalis), of the
pink family is the best-known soap plant in America; it is indigenous to W Asia and Europe but was cultivated in colonial gardens of North America and is now widely naturalized. The lather is obtained from all parts of the plant. The California soap plant or soaproot (
Chlorogalum pomeridianum) of the
lily family is collected in the W United States for its bulb. Other soap plants used locally include an acacia (
Acacia concinna), whose pods are used like the soapberry, and, among American plants, species of
yucca and agave (see
amaryllis), the red buckeye (
Aesculus pavia), the California pigweed (
Chenopodium californicum), the senega snakeroot (
Polygala senega), and species of
Zygadenus and
Ceanothus. The Spanish name
amole is sometimes given to American soap plants, particularly those of the Southwest, where they are most abundant and are still in common use.
